Yeah I think I found it, but its going to the chip that controls the Motor, maybe that chip has that capabilty built in as well. If it was going to an op amp, that means the amps across the shunt are at the micro level?

On newer designs, the op amp is built into the main MCU chip so all you see is maybe a RC filter between the shunt and the MCU.

The shunt resistors are generally in the neighborhood of 1 milliohm. Making an adjustable divider on the signal is pretty easy but most people just lower the shunt resistance and adjust the current limit in software.

thanks very much, I will hack the solder bridge shunt off a damaged controller and try that, for the voltage divider, I am lost trying to find it. i see a trace coming from the shunt towards a gate driver, but seems to be going under the driver to where, difficult to tell

My controller has 6 mosfets, so if I increase the amp capacity of the shunt, I think i must add more mosfets. If I added 6 more mosfets, could I add them in parallel, or does this mod require a new board and controller?

My controller has 6 mosfets, so if I increase the amp capacity of the shunt, I think i must add more mosfets. If I added 6 more mosfets, could I add them in parallel, or does this mod require a new board and controller?

It depends on the FETs and how much current you end up with. Many times you can get away with just cooling the outside of the controller adequately (mounted in the open where there is some air flow).

Can you read the numbers off the existing FETs?

Adding more in parallel makes it hard to cool the added ones. It's usually better to stay with 6 but maybe upgrade them to better ones.

Yes I have some heat sinks out of old PC, was thinking about putting the 6 mosfets on it, but dont know if the leads to the gates etc will be too long. maybe use the PTC or NTC sensor with the arduino or nano to sense when the temp gets too high and power the fan on the heat sink, its out of an old desktop

Those are rated for 55V and have an on resistance of 8 milliohms. For sure you can find FETs with much lower resistance. Less resistance will produce less heating. The legs will melt off the FETs around 70A, so the silicon rating is not the limiting factor.

I'd just try the stock FETs and see if it gets too hot. If they do, then replace them. Any kind of additional heat sinking or air flow will greatly reduce heating also.

for example, on a display panel,where I have my arduino nano display, could a variable pot be used instead of a solder shunt? I guess the resistor values would have to be very small for this to work

Not really. You won't find a variable resistor that can handle that kind of current. You can place a small variable resistor between the shunt and the line going to the MCU. I've done this on several controllers with good results. You have to find the right trace on the board to cut into. If you post good enough pictures of your board, especially the area around the shunt, I can probably find it.

The pictures are not so good. Where the main black battery wire attaches to the board, it goes to one side of the shunt. The other side of the shunt goes to a bunch of the FET legs. The side that goes to the FET legs will have a skinny trace coming off it somewhere that heads to one of the IC chips. It could be on either side of the board. Other than the FET legs, this sensing trace should be about the only thing connected to the FET rail. Ignore any capacitors. Look closely at the board, and if you see something like that, try to get a picture of it. It helps to have lots of light. With some cameras, it might work better to back off a little and zoom in to keep more stuff in focus.

I like the stage light dimmer. That's more like a variable transformer, but the size isn't too far off.

I have outlined the trace in black, it seems to be on the opposite side of the board, this is coming from the underside of the shunt, opposite of the black wire side; seems to be going under the gate driver, or to a pin on it, then onwards to the comparator? then to the MCU

so i dremel tool off some of the trace, then put a pot in between? if its a 3 terminal pot, how do I connect the terminals?

Sorry for the delay, holidays are interfering with the advancement of technology again.

The way you did it will work but you if you turn the pot too far, you can totally defeat the current limiting and possibly blow something up. I've used that method before and it will be OK if you just dial it in carefully and leave it alone after that.

There are many other ways to do this. Below is just one. By using a pot and a fixed resistor, you can limit the maximum increase. In the circuit below, the pot will adjust the current limit from stock level (100%) to 2x (200%). The pot and the fixed resistor are the same value, which gives the 2:1 range. With a different fixed resistor, you could make the range whatever you want.

2x variable shunt divider.jpg (29.79 KiB) Viewed 288 times

I'd like to increase a controller from 25A to ~34A.
I'll use the shunt modification, but like to be a bit precise.
The controller, currently use 2x 5 mohm shunts.

So, please tel me if I'm right :

2x5mohms= 2.5 mohms

2.5 mohms -> 25A
??? mohms -> 34A

From 25A to 34A, it's +36%
So, 2.5 mohms - 36% = 1.6 mohms

3x 5 mohms =~1.67 mohms

I finally just need to add a 3rd 5 milliohms shunt on my controller to reach my goal ?

To get 34A, the shunt would be about 1.84 milliohm. (2.5 x 25/34)

If you add one 5 milliohm to the existing, your limit would be about 37.5A (that's probably close enough)

The "shunt resistor" in my diagram is the one soldered into your board. You are just dividing that voltage before it goes to the MCU.

As current passes through the shunt resistor on the board, a voltage is developed across it. Typical shunt resistors are around 1 milliohm, so you would get 1 mv for every amp. This gets amplified and sent to an A/D converter inside the MCU and software takes it from there.